In the $xy$-plane, the vertices of a triangle are $(-1,3), (6,3)$ and $(-1,-4)$. The area of the triangle is ___ square units.

If $(x_1,y_1)$,

$(x_2,y_2)$ and

$(x_3,y_3)$

are the vertices of a triangle then its area is given by,

$A=\left| \dfrac { 1 }{ 2 } (x_{ 1 }(y_{ 2 }-y_{ 3 })+x_{ 2 }(y_{ 3 }-y_{ 1 })+x_{ 3 }(y_{ 1 }-y_{ 2 })) \right|$

Therefore, with the vertices $(-1,3)$,

$(6,3)$ and

$(-1,-4)$.

Area of triangle is given by,

$A=\left| \dfrac { 1 }{ 2 } (-1(3-(-4))+6(-4-3)+(-1)(3-3)) \right|$

$=\left| \dfrac { 1 }{ 2 } (-7-42) \right|$

$=\left| \dfrac { 1 }{ 2 } (-49) \right|$

$=\left| -24.5 \right|$

$=24.5$ square units.